Electronic and other electrical devices often act unpredictably or may be damaged if operated at low supply voltages. For example, an integrated circuit may be rated for operation at 4.5 Volts, but not at 4.2 Volts. A range of suitable power supply voltage levels may be specified. The integrated circuit may enter an indeterminate or other inappropriate state if the voltage level of the power supply falls below the range. Analog circuits may also exhibit improper behavior if operated at low voltages. Motors may be damaged if operated at less than the rated supply voltage.
A detection circuit is often used to monitor the supply voltage and disable functions if the voltage level of the power supply is less than a minimum required voltage or an undervoltage threshold (e.g., 4.2 Volts for power analog circuits driving motors). The detection circuit monitors the voltage level of the power supply and provides a signal indicative of whether the power supply is adequate. A control circuit then uses the signal to determine whether to disable or enable operation of the electrical devices. Disabling operation of an electrical device is referred to as undervoltage lockout.
The detection circuit may be configured to avoid unnecessary or excessive deactivation of system features. For example, the undervoltage determination often involves a temporal component. Filtering may be used. In this way, very narrow or brief changes (e.g., spikes) in the voltage level of the power supply are often filtered out.
Further attempts to avoid excessive cycling into and out of an undervoltage lockout may involve hysteresis. With hysteresis, the voltage level at which an increasing (rising) supply is deemed adequate may differ from the voltage level, or undervoltage threshold, at which a decreasing (falling) supply is deemed inadequate. The size, or spread, of the difference may be used to decrease how frequently system features are deactivated and reactivated in connection with undervoltage lockout.
The use of hysteresis may undesirably raise the minimum turn on voltage of a system. Different system components may have different minimum voltage supply levels. For example, control circuitry may be operational at voltage levels around 4.5 Volts, while such voltage levels may be insufficient to operate power driver circuits.